Java

Core Java Interview Questions -11

The fractional part of the result is truncated. This is known as rounding toward zero.

2. What method is invoked to cause an object to begin executing as a separate thread?

The start() method of the Thread class is invoked to cause an object to begin executing as a separate thread.
3. What happens when a thread cannot acquire a lock on an object?

If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object’s lock, it enters the waiting state until the lock becomes available.

4. Can a double value be cast to a byte?
Yes, a double value can be cast to a byte.

5. Name two subclasses of the TextComponent class.
TextField and TextArea
6. What is the difference between a break statement and a continue statement?
A break statement results in the termination of the statement to which it applies (switch, for, do, or while). A continue statement is used to end the current loop iteration and return control to the loop statement.

7. How are Java source code files named?
A Java source code file takes the name of a public class or interface that is defined within the file. A source code file may contain at most one public class or interface. If a public class or interface is defined within a source code file, then the source code file must take the name of the public class or interface. If no public class or interface is defined within a source code file, then the file must take on a name that is different than its classes and interfaces. Source code files use the .java extension.

8. What is an abstract method?
An abstract method is a method whose implementation is deferred to a subclass.

9.What must a class do to implement an interface?
It must provide all of the methods in the interface and identify the interface in its implements clause.

10. What is the advantage of the event-delegation model over the earlier event-inheritance model?
The event-delegation model has two advantages over the event-inheritance model. First, it enables event handling to be handled by objects other than the ones that generate the events (or their containers). This allows a clean separation between a component’s design and its use. The other advantage of the event-delegation model is that it performs much better in applications where many events are generated. This performance improvement is due to the fact that the event-delegation model does not have to repeatedly process unhandled events, as is the case of the event-inheritance model.

11. What are the high-level thread states?
The high-level thread states are ready, running, waiting, and dead.

12. Which containers may have a MenuBar?
Frame

13. How are commas used in the intialization and iteration parts of a for statement?
Commas are used to separate multiple statements within the initialization and iteration parts of a for statement.

14. What is the purpose of the wait(), notify(), and notifyAll() methods?
The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to wait for a shared resource. When a thread executes an object’s wait() method, it enters the waiting state. It only enters the ready state after another thread invokes the object’s notify() or notifyAll() methods.
15. What is the difference between the String and StringBuffer classes?
String objects are constants. StringBuffer objects are not.
16. If a variable is declared as private, where may the variable be accessed?
A private variable may only be accessed within the class in which it is declared.

17. What value does read() return when it has reached the end of a file?
The read() method returns -1 when it has reached the end of a file.

18. Can a Byte object be cast to a double value?
No, an object cannot be cast to a primitive value.

19.What is the relationship between the Canvas class and the Graphics class?
A Canvas object provides access to a Graphics object via its paint() method.

20. What is the difference between a Window and a Frame?
The Frame class extends Window to define a main application window that can have a menu bar.

21. Which class is extended by all other classes?
The Object class is extended by all other classes.
22. How are the elements of a BorderLayout organized?The elements of a BorderLayout are organized at the borders (North, South, East, and West) and the center of a container.

23. What is the % operator?
It is referred to as the modulo or remainder operator. It returns the remainder of dividing the first operand by the second operand.

24. What is the difference between a static and a non-static inner class?
A non-static inner class may have object instances that are associated with instances of the class’s outer class. A static inner class does not have any object instances.

25. When can an object reference be cast to an interface reference?
An object reference be cast to an interface reference when the object implements the referenced interface.

26. Can an object be garbage collected while it is still reachable?
A reachable object cannot be garbage collected. Only unreachable objects may be garbage collected..
27. What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?
The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented.

28. Does a class inherit the constructors of its superclass?
A class does not inherit constructors from any of its superclasses.

29. For which statements does it make sense to use a label?
The only statements for which it makes sense to use a label are those statements that can enclose a break or continue statement.
30. Is the ternary operator written x : y ? z or x ? y : z ?
It is written x ? y : z.

31. What is the Map interface?
The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.

32. What is the difference between the Font and FontMetrics classes?
The FontMetrics class is used to define implementation-specific properties, such as ascent and descent, of a Font object.

33. What is the Dictionary class?
The Dictionary class provides the capability to store key-value pairs.

34. What is an object’s lock and which object’s have locks?
An object’s lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object’s lock. All objects and classes have locks. A class’s lock is acquired on the class’s Class object.

35. If a class is declared without any access modifiers, where may the class be accessed?
A class that is declared without any access modifiers is said to have package access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package.

36. What is the SimpleTimeZone class?
The SimpleTimeZone class provides support for a Gregorian calendar.
37. What classes of exceptions may be caught by a catch clause?
A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types.

When a window is repainted by the AWT painting thread, it sets the clipping regions to the area of the window that requires repainting.

40. How are the elements of a Card Layout organized?

The elements of a Card Layout are stacked, one on top of the other, like a deck of cards.

41. What is the purpose of the System class?
The purpose of the System class is to provide access to system resources.

42. Which Text Component method is used to set a TextComponent to the read-only state?
setEditable()
43. Is “abc” a primitive value?
The String literal “abc” is not a primitive value. It is a String object.

44. Which class should you use to obtain design information about an object?
The Class class is used to obtain information about an object’s design.

45. Is &&= a valid Java operator?
No, it is not.

46. What restrictions are placed on the values of each case of a switch statement?
During compilation, the values of each case of a switch statement must evaluate to a value that can be promoted to an int value.

47. What is the relationship between an event-listener interface and an event-adapter class?
An event-listener interface defines the methods that must be implemented by an event handler for a particular kind of event. An event adapter provides a default implementation of an event-listener interface.

48. What is the highest-level event class of the event-delegation model?
The java.util.EventObject class is the highest-level class in the event-delegation class hierarchy.

49. What advantage do Java’s layout managers provide over traditional windowing systems?
Java uses layout managers to lay out components in a consistent manner across all windowing platforms. Since Java’s layout managers aren’t tied to absolute sizing and positioning, they are able to accomodate platform-specific differences among windowing systems.

50. Is a class a subclass of itself?
A class is a subclass of itself.
51. What is the Collection interface?
The Collection interface provides support for the implementation of a mathematical bag – an unordered collection of objects that may contain duplicates.

52.What modifiers can be used with a local inner class?
A local inner class may be final or abstract.

53. What is the difference between a while statement and a do statement?
A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.

54. What modifiers may be used with an interface declaration?
An interface may be declared as public or abstract.

55. What event results from the clicking of a button?
The ActionEvent event is generated as the result of the clicking of a button.

56. How can a GUI component handle its own events?
A component can handle its own events by implementing the required event-listener interface and adding itself as its own event listener.

57. How are the elements of a GridBagLayout organized?
The elements of a GridBagLayout are organized according to a grid. However, the elements are of different sizes and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.

58. What is the difference between static and non-static variables?
A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.
59. Which Math method is used to calculate the absolute value of a number?
The abs() method is used to calculate absolute values.

60. When does the compiler supply a default constructor for a class?
The compiler supplies a default constructor for a class if no other constructors are provided.

61. When is the finally clause of a try-catch-finally statement executed?
The finally clause of the try-catch-finally statement is always executed unless the thread of execution terminates or an exception occurs within the execution of the finally clause.

62. How does multitasking take place on a computer with a single CPU?
The operating system’s task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.

63. What is the difference between the paint() and repaint() methods?
The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to be invoked by the AWT painting thread.

64. Which class is the immediate super class of the Container class?
Component

65. Which non-Unicode letter characters may be used as the first character of an identifier?
The non-Unicode letter characters $ and _ may appear as the first character of an identifier

66. What is the purpose of the File class?
The File class is used to create objects that provide access to the files and directories of a local file system.
67. Can an exception be re thrown?
Yes, an exception can be rethrown.

68. If a method is declared as protected, where may the method be accessed?
A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.

69. How can the Check box class be used to create a radio button?
By associating Checkbox objects with a CheckboxGroup.

70. What restrictions are placed on method overloading?
Two methods may not have the same name and argument list but different return types.

71. What happens when you invoke a thread’s interrupt method while it is sleeping or waiting?
When a task’s interrupt() method is executed, the task enters the ready state. The next time the task enters the running state, an InterruptedException is thrown.
72. What is casting?
There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.
73. Name four Container classes.
Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane

74. What is the difference between a field variable and a local variable?
A field variable is a variable that is declared as a member of a class. A local variable is a variable that is declared local to a method.

75. Under what conditions is an object’s finalize() method invoked by the garbage collector?
The garbage collector invokes an object’s finalize() method when it detects that the object has become unreachable.

76. What is casting?
There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.

77. What is the return type of a program’s main() method?
A program’s main() method has a void return type.

78. What class allows you to read objects directly from a stream?
The ObjectInputStream class supports the reading of objects from input streams.

79. What is the difference between a Choice and a List?
A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices. Only one item may be selected from a Choice. A List may be displayed in such a way that several List items are visible. A List supports the selection of one or more List items.

80. What is the relationship between a method’s throws clause and the exceptions that can be thrown during the method’s execution?
A method’s throws clause must declare any checked exceptions that are not caught within the body of the method.

81. What Check box method allows you to tell if a Checkbox is checked?
getState()

82. What class of exceptions are generated by the Java run-time system?
The Java runtime system generates RuntimeException and Error exceptions.

83. What state is a thread in when it is executing?
An executing thread is in the running state.

84. How is it possible for two String objects with identical values not to be equal under the == operator?
The == operator compares two objects to determine if they are the same object in memory. It is possible for two String objects to have the same value, but located indifferent areas of memory.

85. Why are the methods of the Math class static?
So they can be invoked as if they are a mathematical code library.

86. How are this() and super() used with constructors?
this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

87. If an object is garbage collected, can it become reachable again?
Once an object is garbage collected, it ceases to exist. It can no longer become reachable again.

88. What is the Set interface?
The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.
89. What are the legal operands of the instanceof operator?
The left operand is an object reference or null value and the right operand is a class, interface, or array type.

90. What is the difference between the JDK 1.02 event model and the event-delegation model introduced with JDK 1.1?
The JDK 1.02 event model uses an event inheritance or bubbling approach. In this model, components are required to handle their own events. If they do not handle a particular event, the event is inherited by (or bubbled up to) the component’s container. The container then either handles the event or it is bubbled up to its container and so on, until the highest-level container has been tried.
In the event-delegation model, specific objects are designated as event handlers for GUI components. These objects implement event-listener interfaces. The event-delegation model is more efficient than the event-inheritance model because it eliminates the processing required to support the bubbling of unhandled events.

91. What an I/O filter?
An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.

92. How are the elements of a GridLayout organized?
The elements of a GridBad layout are of equal size and are laid out using the squares of a grid.

93. What is the difference between the File and RandomAccessFile classes?The File class encapsulates the files and directories of the local file system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file.

94. What happens when you add a double value to a String?
The result is a String object.

95. What are E and PI?
E is the base of the natural logarithm and PI is mathematical value pi.

96. What is the purpose of the enableEvents() method?
The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled when a listener is added to an object for a particular event. The enableEvents() method is used by objects that handle events by overriding their event-dispatch methods.

97. What interface must an object implement before it can be written to a stream as an object?

An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.

98. How are this and super used?
this is used to refer to the current object instance. super is used to refer to the variables and methods of the superclass of the current object instance.

99. Are true and false keywords?
The values true and false are not keywords.

100. What is your platform’s default character encoding?If you are running Java on English Windows platforms, it is probably Cp1252. If you are running Java on English Solaris platforms, it is most likely 8859_1..
101. What is a void return type?

A void return type indicates that a method does not return a value.

102. What is the purpose of garbage collection?
The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources may be recla

103. Which package is always imported by default?
The java.lang package is always imported by default.

104. What classes of exceptions may be thrown by a throw statement?
A throw statement may throw any expression that may be assigned to the Throwable type.